I. Field of the Invention
This invention relates to screen printing machines, particularly screen printing machines for use where extremely close tolerances are necessary, for example, in a manufacture of printed circuit boards, printed circuit film, and in the graphic arts and in other manufactures where very close tolerances and/or very fine images are desired on the blanks or work.
II. Description of the Prior Art
Screen printing is an art that generally involves the employment of a platen or table, which has a flat surface supporting a planar blank or piece of "work". A screen, generally comprising a rectangular grid of thin wires, commonly called a silk screen, is brought into overlying relation to the blank, generally not immediately adjacent the blank, but a short distance disposed thereabove. The screen wires are under substantial tension. A stencil has, at this point, already been built into the screen, generally by the use of solidifiable liquids, filling the pores of the holes, where desired, such that when such solidifiable liquids are dried, they define the image on the screen that is to be the stencil for blocking areas where ink applied through the screen to the blank below, will not appear. With the screen into position just above the blank, ink is forced or pumped through the unstenciled openings in the screen to produce a printed pattern on the blank of material therebelow. Generally, the ink is forced through the screen by a squeegee, that is moved in a traversing movement across the screen, and which is the means for pumping the ink through the screen stencil, onto the work, as it presses the screen against the work.
in the electronic industry, printed circuit boards, printed circuit film, and other unyielding, sometimes rigid, and generally non-porous substrates are used. These boards, substrates and the like, generally involve the removal of unprinted metal from a generally uniform metallic surface on the blank. This metal is removed generally by printing resist or masking liquid (generally referred to herein as ink) on localized regions of the blank. After the resist dries, or is solidified by the use of photopolymerizable ink, or heat cured inks and coatings or the like, the boards, films, or the like may be etched or otherwise subjected to removal operations, for removal of metal from the desired (unprinted) localized regions of the metallic surfaces. The electrical characteristics of the work are often determined by the thickness of the metallic layer remaining and its lateral dimensions. Accordingly, the fidelity of the printing when resist is applied to the work, greatly affects the quality of the final printed circuit product. Additionally, in the electronic industry, after the printed circuit board, film, etc., is completely etched and cleaned, it often becomes desirable to cover these items with a cover coating, in order to prevent damage to the electrical circuits, during shipment, during further processing, etc. To this end, it may be desired to use screen printing techniques without a stencil.
In the course of screen printing, the squeegee is generally driven across the screen, but in so doing, it engages the screen and it pushes the screen down a slight amount into engagement with the "work" or substrate. This is so that the inherent resilience of the screen will cause it to be drawn away from the substrate as the blade of the squeegee passes printed portions of the substrate. This pulling away of the screen is referred to as "snap-off". During snap-off, the tacky ink-wetted screen is pulled away from the substrate under the influence of the highly tensioned screen fabric. When tacky inks are used, smearing may become a problem, particularly when printing large solid areas. Additionally, where printing of fine dimensions, such as parallel lines is desired, particularly, again, when tacky inks are used, but even when less tacky inks are used, sharp resolution is negatively affected, and distortions can occur.